1. Field of the Invention
The present invention relates to a laser oscillator output controlling apparatus. More specifically, the invention relates to a laser oscillator employed as a laser source for use in machining operations such as cutting a workpiece.
2. Description of the Background Art
A conventional laser oscillator output controlling apparatus is shown in FIG. 9. In such apparatus, a part of the output 3 from laser oscillator 1 is diverted by a partial reflector 2 so that the intensity of the laser output 3 can be detected by a laser output detector 4. An error amplifier 6 compares an external intensity command 5 with the intensity of the laser output as detected by detector 4 and amplifies the resulting error value for input to the laser oscillator 1, thereby controlling the laser oscillator 1 to produce a laser output 3B with a desired intensity level.
Since the laser output detector 4 includes a thermocouple, the controlling apparatus is slow in responding to a change in external intensity command 5. To overcome this problem, a laser output controlling apparatus as shown in FIG. 10 has been used. Such a conventional laser output controlling apparatus is disclosed, for example, in Japanese Patent Disclosure Publication No. 202794/1982, and U.S. Pat. No. 3,806,829.
As shown in FIG. 10, the controlling apparatus is composed of a laser oscillator 1, a partial reflector 2 for diverting a fractional part (e.g., 1 or 2%) of laser output 3 of the laser oscillator 1, a laser output detector 4 for detecting the intensity of the diverted beam 3A, an external intensity command 5 for controlling the intensity of the laser output 3, a processing unit 7 for receiving the output of the laser output detector 4 and the external intensity command 5, and a storage device 8 for storing the input/output characteristic (pattern) of the laser oscillator 1. In operation, the pattern stored in the input/output pattern storage device 8 is used by the processing unit 7 to determine an input laser oscillator signal based on the output of the laser output detector 4 and the command 5. The processing unit 7 outputs the resulting output pattern to the laser oscillator 1 as a laser oscillator input signal 9, thereby causing the laser output 3 of the laser oscillator 1 to change until it reaches the desired intensity value represented by the external intensity command 5.
The operation of the known laser output controlling apparatus arranged as described above will now be described in more detail with reference to FIG. 11(a), which illustrates the input/output characteristic of the laser oscillator 1 as a line through points a and b.
Assuming that the desired intensity of the laser output as represented by external command 5 is Ya, the required input (i.e., laser oscillator input signal 9) value of the laser oscillator 1 according to the input/output characteristic would be Xa. In a steady-state condition, the value of the external command 5 and the value of the laser output 3 are equal, i.e., both have axis value Ya. However, when the command 5 is suddenly changed to Yb, the processing unit 7 must now produce an output value Xb based on the pattern stored in storage device 8 and the command value Yb. By changing the laser oscillator input signal 9 of the laser oscillator to Xb the laser oscillator 1 is controlled to change the laser output 3 to the new desired value Yb.
In the above description, the line drawn through points a and b represents the normal input/output characteristic pattern of the laser oscillator 1. When the normal characteristic pattern has changed, however, the characteristic shifts to the line connecting points a' and b' in FIG. 11(a). Because of this change, the pattern stored in storage device 8 will not provide processing unit 7 with accurate characteristic data to produce a laser oscillator input signal. To overcome this problem, processing unit 7 automatically compensates for the shift so that the laser output 3 can be controlled to output the desired intensity irrespective of the shift in characteristics. This shift, however, must be detected prior to operation because the conventional apparatus does not perform the compensation in a case where the shift occurs during operation (e.g., while cutting a workpiece). In the conventional apparatus, no consideration is given to control in an on-line mode (i.e., when a workpiece is actually being cut using the laser beam output by the laser oscillator). Thus, it may be difficult to apply this type of controlling apparatus on-line.
The conventional laser output controlling apparatus arranged as described above has many other disadvantages. For example, the laser output detector 4 cannot accurately detect the value of the laser output 3 at the time the input signal 9 is changed (e.g., from Ya to Yb) because the detector employed has an inherent delay in responding to change in laser output. Specifically, when laser oscillator input signal 9 is received, laser oscillator 1 produces laser output 3. However, the speed of response of laser output 3 to the laser oscillator input signal 9 is quicker than the response speed of laser output detector 4 (i.e., the response speed of laser output detector 4 to laser output 3). When a desired value of output command 5 is given to processing unit 7, processing unit 7 refers to the input/output characteristic of the laser oscillator 1 stored in the output pattern storage device 8 and reads the value of laser oscillator input signal 9 corresponding to a desired value of output command 5 and sends it to the laser oscillator 1 as laser oscillator input signal 9. Specifically, this laser oscillator output controlling apparatus will not feedback control the laser output 3 to be a desired value of output command 5, based on the laser output value detected by laser output detector 4, but as described above, it will refer to an input/output characteristic of laser oscillator 1 stored in output pattern storage device 8 and will read the value of laser oscillator input signal 9 corresponding to a desired value of output command 5 and will send it to the laser oscillator 1. It is not a feedback control. And furthermore, as described above, response speed of laser output 3 to laser oscillator input signal 9 is so fast as to be almost instantaneous. Therefore, even if laser output detector 4 cannot accurately detect the value of laser output 3, even if a detector which is slow in responding is used, laser oscillator input signal 9 can be set irrespective of the output of laser output detector 4 by processing unit 7 and can control laser output 3 at high speed.
Also, while the normal (expected) input/output characteristic of the laser oscillator in FIG. 11(a) is linear, the actual input/output characteristic of the laser oscillator 1 may be one of a plurality of complicated curves, as shown in FIG. 11(b). The actual input/output characteristic of the laser oscillator 1 departs from the linear representation shown in FIG. 11(a) because of the effects of aging and the like. For example, as shown in FIG. 11(b), the input/output characteristics of a gas closed-type carbon dioxide (CO.sub.2) laser oscillator changes with the increase of time t. As the laser oscillator ages, the intensity of the output decreases for a given amount of excitation so that it is necessary to increase the excitation (i.e., laser oscillator input signal value) in order to compensate for the decreasing intensity caused by aging. Thus, when time passes from t.sub.0 through t.sub.3 the threshold excitation values (i.e., the values in which the laser output is generated) move towards the right and each subsequent characteristic curve has a gentler slope. Thus, as the laser oscillator 1 ages, the actual input/output characteristic departs from the normal input/output characteristic. Without compensating for this departure, a large error may occur in using the laser oscillator as a source in a machining operation or the like.
Moreover, the conventional apparatus is not provided with a function capable of indicating in a certain form that the input/output characteristic of the laser oscillator diverges significantly from a normal (expected) input/output characteristic, and thus the existence of such divergence is not known to an operator, thereby posing a problem in practical use. Also, the use of the laser apparatus under such divergent conditions may prevent adequate protection of oscillator energizing power supply hardware.